The so-called methanation reaction, in which carbon monoxide and hydrogen are reacted to produce methane, has been conventionally utilized for the purpose of converting carbon monoxide, which is a poison to the catalysts used for an ammonia synthesis reaction, to methane, which does not poison the ammonia synthesis catalyst. Recently, the use of the methanation reaction as a means for producing a gaseous mixture enriched with methane, a fuel gas for industrial or domestic use, has been studied. The gaseous mixture enriched with methane is produced from a starting gaseous mixture consisting essentially of carbon oxides and hydrogen produced by the gasification of lower grade fuels, such as coal or heavy oil.
The methanation reaction employs hydrogen and either or both of carbon monoxide and carbon dioxide as starting materials, and proceeds, in the presence of a suitable catalyst, in accordance with the following equations: EQU CO+3H.sub.2 =CH.sub.4 +H.sub.2 O (1) EQU CO.sub.2 +4H.sub.2 =CH.sub.4 +2H.sub.2 O (2) EQU CO+H.sub.2 O=CO.sub.2 +H.sub.2 ( 3)
All of these reactions, of which reaction (1) is generally considered the main reaction and the reactions (2) and (3) are considered side reactions, do not proceed satisfactorily toward the right side of the respective equations due to the chemical equilibria established when the reaction temperature is high. Moreover, the reactions according to the equations (1) and (2), as is known in the art, are accompanied by generation of a very large amount of heat (exothermic reaction heat).
Several processes are disclosed in U.S. Pat. No. 4,130,575, British Pat. No. 1 516 319, and Japanese Pat. Laid-Open No. 82188/1980 for effecting methanation according to the foregoing reactions on a large scale. All of the foregoing prior art processes are two-stage reaction processes wherein an adiabatic reactor is used as a first reactor to which a feed gas is supplied, and the feed gas, after having undergone partial methanation in the first reactor, is then flowed through a second reactor to complete the methanation reaction. In these known processes, the amount of reaction heat generated in each of these reactors is controlled by recirculating a considerable portion of the reaction product gas, which is discharged from the catalyst bed of the first or second reactor and then cooled, to the inlet of the catalyst bed of the first reactor.
In these known processes, some of the heat of reaction is lost when the reaction product gas is cooled and recirculated, and it is further necessary to use a second reactor for the purpose of further effecting the methanation reaction and thereby lowering the content of carbon monoxide in the final product gas. The carbon monoxide content of the product gas produced by the first reactor used in these prior art processes is high due to the high temperature of the adiabatic catalyst bed of the first reactor, and to the chemical equilibrium relationships established by the foregoing three chemical reactions at such high temperature. Accordingly, these conventional methanation processes involve the drawbacks that a large part of the reaction heat cannot be recovered in the form of valuable high temperature, heat energy and that complex process equipment is required.